signal splitting method for limiting peak power in a CDMA system

ABSTRACT

The invention is a method for limiting the peak transmit power in a CDMA communication system. At least one of first and second high transmit power regions are separated into a plurality of high transmit power subregions. The high transmit power subregions of the plurality of high subregions are shifted by time offsets of differing durations to provide a plurality of time offset subregions. First and second low transmit power regions are also provided. At least one of the first and second low transmit power regions is also separated into a plurality of transmit power subregions and the low transmit power subregions are shifted by time offsets of differing time durations. The subregions can be time offset by a predetermined time duration or by a random time duration.

CROSS REFERENCE

[0001] This divisional application claims priority from co-pendingapplication Serial No. 09/144,408, filed Aug. 31, 1998, entitled “SignalSplitting Method for Limiting Peak Power in a CDMA System,” and assignedto the assignee of the present invention.

BACKGROUND OF THE INVENTION

[0002] I. Field of the Invention

[0003] The present invention relates to communication systems in generaland, in particular, to improving the transmission of information signalsin a communications system.

[0004] II. Description of the Related Art

[0005] CDMA communication systems are very sensitive to peak transmitpower and are generally limited by interference related to transmitpower levels. One interference related limitation is the so called“Near-Far Problem”. In this problem as transmit power increases during atransmission it causes more interference in other channels. To deal withthis additional interference the other channels must increase their owntransmit power. The increase in transmit power by the other channels inturn generates more interference for all the channels. This avalancheeffect occurs until the system is stabilized and all the channels aresatisfied. Therefore, in order to maximize the capacity of such a systemit is desirable that each user transmit only the minimum power necessaryto achieve a required quality of service. Another problem that candegrade the performance of other links in a transmission system is awaveform that contains a discontinuous power pattern. This problemcompounds the Near-Far Problem.

[0006] Transmit power amplifiers provide another area where interferencecan limit the capacity of CDMA communication systems. The maximum outputpower of transmit power amplifiers is determined by a number of designparameters including power dissipation and unwanted emissions. Unwantedemissions are those that are outside the bandwidth of the input signal.Most of the unwanted emissions occur due to intermodulation within thepower amplifier. Intermodulation is caused by high transmit power levelsthat drive the amplifier into a nonlinear region.

[0007] Unwanted emissions are often limited by regulatory bodies, suchas the FCC. Industry standards may also set limits on unwanted emissionsin order to avoid interference with the same or another system. Tomaintain unwanted emissions within the desired limits, the output powerof the transmit power amplifier is selected so that the probability ofexceeding the emission limits is very small. When a waveform having anonlinear envelope is amplified, the maximum output is determined by theportion of the waveform that has the highest power level. Additionally,if the requested output power exceeds the maximum permitted outputpower, a transmitter can limit the output power to the maximum permittedlevel in order to keep the unwanted emissions within the prescribedlimits.

[0008] Referring now to FIG. 1, there is shown graphical representation10 of transmission waveforms 12, 18. Transmission waveform 12 is formedof waveform portions 14, 16 having differing power levels. The transmitpower level limitation of the amplifier is will be reached by portion 14rather than by portion 16 because portion 14 has the highestinstantaneous power. In contrast, transmission waveform 18 has aconstant envelope. Transmitting at the maximum power permits higherenergy transmission, as illustrated by the areas under transmissionwaveforms 12, 18. In order to maximize the total transmit energy over aperiod of time it is therefore desirable that the signal applied to thetransmitter have a peak to average power ratio as close to one aspossible. Furthermore, in addition to preventing the peak transmit powerproblems, a constant power level reduces self interference that canresult from fast changes of the loading in the power amplifier.

[0009] For example, FIG. 2 shows a plurality of transmission waveforms20 a-n. The number n of transmission waveforms 20 a-n can be very large.For example, n can commonly have a value of two hundred or more in CDMAcommunication systems. Transmission signal 20 a-n is formed of pilotportions 22, control portions 24, voice portions 26, and data portions28. Pilot portions 22 of transmission signals 20 a-n always have a highpower level. By definition, in order to serve as a pilot signal, portionportions 22 must always be high. Data portions 28 are usually relativelyhigh because it is a very highly utilized time slot. Voice portions 26,on the other hand, are typically low because voice signals have manyunused periods.

[0010] Total power waveform 30 represents the total power oftransmission waveforms 20 a-n summed together. Because pilot portions 22and data portions 22 are at high levels within transmission waveforms 20a-n, the corresponding portions 32, 36 of total power waveform 30 arehigh. Because voice portions 26 vary and are usually low, portion 34 oftotal power waveform 30 can vary from close to zero to an intermediatelevel 34.

SUMMARY OF THE INVENTION

[0011] The invention is a method for limiting the peak transmit power ina CDMA communication system. At least one of first and second hightransmit power regions are separated into a plurality of high transmitpower subregions. The high transmit power subregions of the plurality ofhigh subregions are shifted by time offsets of differing durations toprovide a plurality of time offset subregions. First and second lowtransmit power regions are also provided. At least one of the first andsecond low transmit power regions is also separated into a plurality oftransmit power subregions and the low transmit power subregions areshifted by time offsets of differing time durations. The subregions canbe time offset by a predetermined time duration or by a random timeduration.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] The features, objects, and advantages of the present inventionwill become more apparent form the detailed description set forth belowwhen taken in conjunction with the drawings in which like referencecharacters identify corresponding elements throughout and wherein:

[0013]FIG. 1 shows a graphical representation of transmission waveforms;

[0014]FIG. 2 shows a plurality of transmission signals in acommunication system;

[0015]FIG. 3 shows a graphical representation of a transmissionwaveform;

[0016]FIG. 4 shows a graphical representation of transmission waveforms;

[0017]FIG. 5 shows a graphical representation of transmission waveforms;

[0018]FIG. 6 shows a flowchart representation of an algorithm forpredicting the peak transmit power level in a CDMA system; and

[0019]FIG. 7 shows a graphical representation of a transmission waveforminterleaved according to the method of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0020] Referring now to FIG. 3, there is shown a graphicalrepresentation of transmit waveform 50. A large number of waveforms suchas transmit waveform 50 are conventionally transmitted simultaneously inCDMA communication systems. Transmit waveforms 50 are formed of aplurality of slots 54. Within each slot 54 are three regions havingpower levels A, B, and C. If a number of transmit waveforms 50 aretransmitted through a communication band in such a way that power levelsA of the various waveforms 50 occur simultaneously, the total powertransmitted through the band reaches a peak at that time. Likewise, iftransmit waveforms 50 are transmitted such that power levels C occursimultaneously, the total power of the band reaches a low level at thattime.

[0021] However, in a preferred embodiment of the present inventiontransmit waveforms 50 are time offset with respect to each other in sucha way that the high power levels A do not line up with each other. Inthis way the high levels and the low levels of the various transmitwaveforms 50 are averaged out. This results, most importantly, in alower peak transmit power in the communication band. As previouslydescribed, a lower peak transmit power reduces unwanted emissions andinterference.

[0022] Referring now to FIG. 4, there is shown graphical representation70 of transmit waveforms 74 a-n. Transmit waveforms 74 a-n can includepilot portions 78, power up/down portions 82, control portions 86, anddata portion 90 within each time slot 72. Data portions 90 contain datapulse 92. The peak transmit power of a band carrying transmit waveforms74 a-n is the sum of the power of each waveform 74 a-n. Thus, in orderto minimize the peak transmit power, and to thereby minimize unwantedemissions, the sum of transmit waveforms 74 a-n can be averaged andsmoothed.

[0023] In one preferred embodiment of the invention, the averaging ofthe high transmit levels A of transmit waveforms 74 a-n is accomplishedby providing each successive waveform 74 a-n with the same fixed offsetwhen a new waveform 74 a-n is added to the communication band. Thus, forillustrative purposes, transmit waveforms 74 a-n are identical to eachother except that they are time offset from each other by differingmultiples of the fixed time offset to.

[0024] For example, if transmit waveform 74 a is the first signal to betransmitted by a communication band, it can be transmitted with zerooffset. If transmit waveform 74b is the next signal to be transmittedwithin the communication band it can receive time offset t_(o) withrespect to transmit waveform 74 a. If transmit waveform 74 c is the nextsignal to be transmitted it can be time offset by t₀ with respect totransmit waveform 74 b. This is equivalent to a time offset of 2t₀ fromwaveform 74 a. Each subsequent transmit waveform 74 a-n to betransmitted by way of the communication band can then receive anadditional offset t₀ in the same manner. It will be understood howeverthat it is not always possible to shift every waveform by any timeoffset that may be required by this method.

[0025] Referring now to FIG. 5, there is shown graphical representation100 including transmit waveform 74 and total transmit power waveform 96.When practicing the method of the present invention, further averagingof transmit waveforms 74 a-n, and therefore further improvement in thepeak transmit power, can be obtained by smoothing data pulse 92 withindata portion 90 of waveforms 74 a-n prior to applying time offsets. Inorder to obtain this further improvement, conventional techniques fordistributing the information of data pulse 92 throughout data portion 90can be used. Additionally, the position of data pulse 92 within dataportion 90 can be varied in order to minimize the peak transmit power.Using these methods a transmit power level 94 can result within in totaltransmit power waveform 96.

[0026] In another embodiment of the present invention, the variousportions within time slots 72 of transmit waveforms 74 a-n can beseparated from each other and transmitted in any of the possiblesequences. For example, within time slot 72 data portion 90 can beseparated from the remainder of transmit waveform 74 a and transmittedfirst. Pilot portion 78 can be separated and transmitted next after dataportion 90. The remaining portions within time slot 72 can also betransmitted in any sequence. Applying this technique to the waveform ofgraphical representation 50, portions A, B, and C can be transmitted asABC, ACB, or in any other order. Furthermore, the sequences can bevaried from one transmit waveform 74 a-n to the next

[0027] Improved results can be obtained in the method of separating andreordering the portions of transmit waveforms 74 a-n by randomlychanging the sequence of the transmissions of the waveform portions.This results in further averaging and smoothing of the contributions tothe total transmit power made by the various waveforms. New transmissionsequences can be continuously produced by a random number generator. Inthis case both the transmitter and the receiver must have knowledge ofthe parameters of the random number generator in order to permitdecoding by the receiver.

[0028] In addition to using a fixed time offset t_(o) for each newwaveform, it is possible to select an individual offset for each newwaveform according to an algorithm. For example, the new time offset canbe selected by determining which of the possible offsets is being usedby the lowest number of existing calls. Additionally, the individualoffsets can be determined by a peak power algorithm adapted to provide aminimum increase in the peak transmit power according to the shape orexpected shape of the new transmission signals. The algorithm can be aheuristic one. In order to perform this function the peak powerminimization algorithm must be able to predict the transmit powerwaveform over a period of time, for example over a transmit frame.

[0029] Referring now to FIG. 6, there is shown transmit power predictionalgorithm 120. Transmit power prediction algorithm 120 can be used topredict the new total power resulting from the addition of, for example,each transmission waveform 74 a-n to a communication system.Additionally, algorithm 120 can be used to predict a new total power foradding a transmission waveform 74 a-c at each of a number of possibletime offsets. Thus, it is possible to select the optimum time offsetresulting in the minimum increase in peak transmit power. By determiningthe optimum time offset for each new transmit waveform 74 a-n as it isadded to the communication system in this manner further improvement insystem performance is obtained in an heuristic manner.

[0030] For example, the total transmit power of some known systems canbe calculated as:

{overscore (P)}=α{overscore (P)} _(n-1)+(1−α){overscore (e)} _(n)

[0031] where:

(1−α)<1

[0032] is the forgetting factor, {overscore (P)}_(n) is the vector withthe frame power estimate at time n with elements {overscore (P)}_(n)′corresponding to the estimated power during the ith symbol in the frame,and {overscore (e)}_(n) is the vector containing the measured power fora frame at time n.

[0033] When a new channel set up is required in order to add a newtransmission waveform, the base station can compute the transmit powerwaveform W resulting from the addition of the new channel. The basestation can then compute the resulting power vectors corresponding toeach of the possible time offsets as follows:

({overscore (P)} _(n)′)_((k)) ={overscore (P)} _(n)+cycl_(k)(W)

[0034] where cyclk( ) is an operator that produces a cyclic shift of thevector W by k elements. The new channel can then be set up with the timeoffset that corresponds to the ({overscore (P)}_(n)′)_((k)) having thepeak power to average power ratio closest to one.

[0035] It will be understood that when a waveform such as transmissionwaveform 50 is separated into sections having power levels A, B and C,the transmission sequence of the sections can be selected in a similarheuristic manner. For example, the resulting peak transmit power can bedetermined for each possible transmission sequence and the transmissionsequence resulting in the lowest peak transmit power can be selected.

[0036] Referring now to FIG. 7, there is shown graphical representation130 of transmit power waveform 132. It is understood by those skilled inthe art that each region A, B and C of representation 50 can beseparated into subregions. The subregions of each region can be as smallas desired, with subregions having a single symbol being permitted. Thesubregions formed by dividing the regions in this manner can then beinterleaved with respect to each other in order to form transmit powerwaveform 132. Additionally, one region of the transmission waveform canbe left intact while the remaining regions can be interleaved. This isset forth as transmit power waveform 134.

[0037] The order of the transmission of the interleaved subregions canbe a predetermined order, a random order, or any other order understoodby those skilled in the art. Separation and interleaving of transmissionwaveforms in this manner provides excellent averaging of transmissionwaveforms and minimizing of peak transmit power. When regions within atransmit power waveform are interleaved in this manner the receiver mustwait for the end of a slot before it can begin decoding.

[0038] The previous description of the preferred embodiments is providedto enable a person skilled in the art to make or use the presentinvention. The various modifications to these embodiments will bereadily apparent to those skilled in the art, and the generic principlesdefined herein may be applied to other embodiments without the use ofthe inventive faculty. Thus, the present invention is not intended to belimited to the embodiments shown herein but is to be accorded the widestscope consistent with the principles and novel features disclosed. Itwill be understood that all of the methods disclosed herein can be usedat the time of call set up or at any time during a transmission afterset up.

[0039] Additionally, it will be understood that the various methods canbe combined with each other in any manner. In particular, all of theseparable waveform methods can be used independently or in conjunctionwith the previously described time shifting based methods, with orwithout the random or heuristic methods. Furthermore, the variousmethods disclosed herein can be performed either at the time of callsetup or at any time during transmission of the transmission waveforms.

1. A method for limiting peak transmit power in a wireless communicationsystem, comprising: providing a transmit waveform; selecting a non-fixedindividual time offset for the waveform according to an algorithm; anddelaying transmission of the waveform by the non-fixed time offset. 2.The method for limiting peak transmit power of claim 1 wherein thealgorithm comprises selecting a time offset by determining which of apossible set of offsets is being used by a lowest number of waveforms.3. The method for limiting peak transmit power of claim 1 wherein thealgorithm comprises a peak power algorithm adapted to provide a minimumincrease in the peak transmit power according to the shape or expectedshape of the transmit waveform.
 4. The method for limiting peak transmitpower of claim 1 wherein the algorithm comprises a heuristic algorithm.5. A system for limiting peak transmit power in a wireless communicationsystem, comprising: means for providing a transmit waveform; means forselecting a non-fixed individual time offset for the waveform accordingto an algorithm; and means for delaying transmission of the waveform bythe non-fixed time offset.
 6. The system for limiting peak transmitpower of claim 5 wherein the means for selecting a non-fixed individualtime offset for the waveform according to an algorithm comprises a meansfor selecting a time offset by determining which of a possible set ofoffsets is being used by a lowest number of waveforms.
 7. The system forlimiting peak transmit power of claim 5 wherein the means for selectinga non-fixed individual time offset for the waveform according to analgorithm comprises a means for a peak power algorithm to provide aminimum increase in the peak transmit power according to the shape orexpected shape of the transmit waveform.
 8. The system for limiting peaktransmit power of claim 5 wherein the means for selecting a non-fixedindividual time offset for the waveform according to an algorithmcomprises a means for performing a heuristic algorithm.